Various systems have been proposed and are in use for scrambling television video signals to secure transmission of video information. Generally, these systems include a scrambler which alters the video signals in a predetermined manner before the video signals are propagated through a video channel using, for example, an RF, microwave, cable or other video communications system. At the receiving end of the video system, a descrambler reverses the scrambling process to assemble the scrambled video signals into their original order, sequence or state so that they can be suitably displayed for viewing on a television screen. Of course, if a receiver does not contain a suitable descrambler, the displayed video signals will be generally unintelligible and the information therein will be masked.
Thompson, et al., U.S. Pat. No. 4,716,588, discloses a multimode video signal scrambling system controlled by a central computer facility which scrambles each field of a video signal using one or more of several techniques including adding extraneous and confusing synchronizing (sync) pulses onto the video signal, compressing the amplitude of the video signal, alternatively inverting horizontal lines of the video signal in time to reverse the sequence thereof, removing horizontal sync pulses from the video signal, adding false leading edge sync pulses to horizontal lines of a video signal and shifting the amplitude of the active video information within a video signal in a predetermined direction so that a television receiver does not register or lock onto the active video information.
Ryan, U.S. Pat. No. 4,916,736, discloses a video scrambling system which shifts active video information in time with respect to the horizontal sync pulse and the color burst signal within each horizontal line of the video signal. This system includes an analog-to-digital (A/D) converter which digitizes and stores the video signal in a memory and a digital-to-analog (D/A) converter which reads the video information out of the memory for transmission through a communications channel. A controller controls the sequence in which the information is read out of the memory and shifts the digitized active video information in time with respect to the horizontal sync pulse and the color burst signal in order to scramble the video signal.
Other methods of scrambling a video signal use time compression and expansion techniques. It is generally known, for example, to time compress or time expand a communications signal and, thereby, expand or compress the bandwidth of that communications signal, prior to the transmission of the signal through a channel. As an example, Nishimura, U.S. Pat. No. 4,742,546, discloses a voice communications system which alternatively subjects a voice signal to time compression and time expansion before delivering the voice signal to a transmitting antennae. A demodulating circuit descrambles the received voice signal by time compressing and/or time expanding the scrambled voice signal in a manner complimentary to the manner of scrambling.
Southworth, U.S. Pat. No. 3,683,111, discloses a system which compresses the bandwidth of a television signal prior to transmission. This system includes a television scan converter which uses high order sampling techniques to sample predetermined portions of a video signal to convert the video signal into a narrowband signal which can be transmitted through a narrowband channel. A receiver converts the narrowband signal back into a standard television signal for display on a television screen. This system is particularly adapted for the detection of moving objects in the field of a view of a camera and is particularly useful with, for example, television surveillance systems which transmit television signals through a narrowband channel such as a channel commonly associated with telephone lines. This system is considered inadequate for commercial video signal scrambling, however, because it samples only portions of the original television video signal and, therefore, loses important video information. Furthermore, this system does not provide the clarity and resolution required in standard commercial television systems.
Dischert, et al., U.S. Pat. No. 4,376,957, discloses a time-division multiplexing apparatus which sequentially transmits multiple time-compressed video signals through a single channel. This system stores video signals in first and second memories at a first data rate and reads the video signals out of the memories at a second and higher data rate to produce time-compressed, bandwidth-expanded television signals which are alternatively delivered to a transmitter for transmission through a single channel. A receiver separates the time-compressed video signals, stores them in memories at the higher data rate and reads them out of the memories at the lower data rate to produce the original video signals. This system is considered inadequate for scrambling commercial video signals, however, because it increases the required bandwidth of the channel and thereby decreases the number of channels possible in any bandwidth limited communications system. Furthermore, increasing the bandwidth of a channel in a commercial, free-propagating television transmission system is undesirable and/or unacceptable because the bandwidths of these channels are limited by law. Increasing the bandwidth of a transmission channel is also unacceptable in CATV systems which maximize the number of channels by forcing the bandwidth of each channel to be as narrow as possible.